An adaptive-optical element typically is configured to have a deformable reflective surface to control and shape a wavefront of light reflected from the surface. Typically, adaptive-optical elements are used to improve the optical resolution of a particular optical system by compensating for fabrication errors (e.g., misshapen or thermally deformed elements) or optical-path aberrations (e.g., atmospheric effects). Such optical elements and systems comprising them achieve this improved performance by shaping the deformable surface in such a way that unwanted aberrations in the incident wavefront are removed. Hence, adaptive-optical elements (e.g., mirrors) are of great interest for use in astronomical telescopes (especially earth-bound telescopes) subject to image-degrading effects caused, for example, by passage of starlight through the earth's atmosphere.
Another area in which interest in adaptive optics is increasing rapidly is optical lithography, especially extreme ultraviolet (EUV) lithography (EUVL) in which all the optical elements used for illumination and image projection are reflective. The performance of an EUVL system can be degraded substantially by manufacturing errors in one or more of the reflective-optical elements such as used in the projection-optical system. Other sources of image degradation include thermally induced deformations in illuminated regions of the lithography systems, vibrations, and image-placement errors and/or distortion errors that occur during lithographic pattern transfer. In a conventional EUVL system utilizing adaptive optics, at least one optical element of the system is configured with a deformable mirror surface. Deformation of the mirror surface is achieved in a controlled manner by an array of actuators that are controlled and energized independently. Unfortunately, the number of individual actuators required for satisfactory performance tends to be quite large, resulting in an excessively crowded array of actuators associated with the mirror. This crowding tends to limit the area of the mirror surface that can be controlled and/or the resolution with which controlled deformation can be achieved. Moreover, because of the relatively high number of individual actuators utilized, conventional actuator arrays for use in these mirrors tend to be very expensive. Accordingly, there is a need for improved actuator arrays for use in adaptive-optical systems.